Existing communication networks include conventional Second Generation (2G) communication networks, 3rd Generation (3G) communication networks, and Long Term Evolution/System Architecture Evolution (LTE/SAE) networks integrated with more advanced technologies. A communication network is generally formed by a Radio Access Network (RAN) and a Core Network (CN). Different communication networks adopt different Radio Access Technologies (RATs) to access the CN, that is, different communication networks have different RANs. For example, a RAN of a 3G communication network is referred to as a Universal Terrestrial Radio Access Network (UTRAN), and a RAN of a SAE communication network is referred to as an Evolved UTRAN (E-UTRAN).
The RAN is formed by RAN nodes, for example, a Radio Network Controller (RNC) and a base station (NodeB) in the UTRAN; and the CN is formed by CN nodes, for example, a Serving General Packet Radio Service (GPRS) Support Node (SGSN) in the UTRAN, or a Mobility Management Entity (MME) in the E-UTRAN. Functions of the MME are to store mobility management context of a user equipment (UE), briefly referred to as UE context, for example, UE identity, mobility management state, and location information.
In the prior art, a RAN node may be routed to a plurality of CN nodes, that is, the RAN node may route an initially accessing UE to different CN nodes. The CN nodes form a Pool. The CN nodes, for example, the SGSNs, in a Pool in a 2G/3G system are identified by Network Resource Identifiers (NRIs), and the CN nodes in a Pool in an SAE system are identified by Globally Unique Mobility Management Entity Identifiers (GUMMEIs).
When a UE accesses a communication network, the communication network allocates a temporary identity to the UE, for example, a 2G/3G system allocates a Packet Temporary Mobile Subscriber Identity (P-TMSI)/TMSI to the UE, while an SAE system allocates a Global Unique Temporary Identity (GUTI) to the UE, where the GUTI contains a GUMMEI. Since the UE may move between different communication networks, when the UE is handed over from the original communication network to a new communication network, or handed over from the original CN node to a new CN node, the original CN node needs to be found by using the NRI/GUMMEI and the temporary identity of the UE to obtain the context of the UE, for achieving a rapid handover. For a handover between different communication networks, since the communication networks adopt different RATs, when the UE is handed over from the original communication network to a new communication network, the identify of a RAT of the original communication network, that is, the identity of an old RAT, also needs to be mapped to the identify of a RAT of the new communication network for access, so that the CN node in the original communication network can be found, and the UE context can be obtained.
Specifically, a GUTI has the following structure: GUTI=GUMMEI+M-TMSI (MME-TMSI); where GUMMEI=PLMN-id+MMEI, PLMN-id=MCC+MNC; and MMEI=MME Group ID+MMEC; therefore, the following equations can be obtained:                GUTI=MCC+MNC+MME Group ID (16 bit)+MMEC (8 bit)+M-TMSI (32 bit);        GUMMEI=MCC+MNC+MME Group ID+MMEC;        MMEI=MME Group ID+MMEC;        S-TMSI (40 bit)=MMEC+M-TMSI        
When the UE carrying the 2G/3G temporary identity P-TMSI/old Routing Area Identifier (RAI) accesses the SAE system, the 2G/3G temporary identity needs to be mapped to a GUTI, as shown in FIG. 11, that is, the Mobile Country Code (MCC) and the Mobile Network Code (MNC) in the old RAI needs to be mapped to an MCC and an MNC in the GUTI respectively, the Location Area Code (LAC) needs to be mapped to a Mobility Management Entity Group Identity (MMEGI) in the GUTI, the NRI needs to be mapped to a Mobility Management Entity Code (MMEC) in the GUTI, and the Routing Area Code (RAC) and other P-TMSI contents needs to be mapped to an M-TMSI in the GUTI.
In the implementation of the present invention, the inventor finds that the prior art has at least the following problems.
Since the mapped RAT identity is different from the real RAT identity, it is possible that no CN node corresponding to the mapped RAT identity will be found, that is, it is possible that no new CN node can be selected in the new communication network.
In addition, since different communication networks are involved, if the new CN node cannot identify whether the current RAT identity is the real RAT identity or the mapped RAT identity, it cannot be known which form of RAT can be used to obtain the UE context from the original CN node, resulting in that the UE context cannot be obtained.